Generally, a flap door for a vehicle is a door apparatus provided in the vehicle so as to be openable and closable in order to put a cargo or a battery in the vehicle or check an engine. The flap door is usually opened and closed in a vertical direction by rotation. To this end, the flap door is connected to a vehicle body through a link structure.
FIG. 1 is a view showing a flap door for a vehicle according to the related art; and FIGS. 2A to 2C are views showing an operation of the flap door for a vehicle according to the related art. Referring to FIG. 1, a door body 200 opens and closes an aperture frame 100 of a space (represented by dot-dashed lines in FIG. 1) in which a cargo is stored. To this end, the flap door has a gas lift 110 having one end fixed to both side walls forming the above-mentioned space and the other end directed toward the door body 200 and extendable in a length direction, and has a link structure vertically rotating the door body 200 by an extension operation of the gas lift 110 in the length direction.
The link structure includes a shaft 120 rotatably installed at a fixed bracket 150 fixed to upper portions of both side walls, an arm link 130 having one end fixed to the shaft 120 and the other end rotatably connected to the gas lift 110, and a rotation link 140 having one end fixed to the shaft 120 and the other end fixed to a lower portion of the door body 200.
In the flap door for a vehicle according to the related art, as shown in FIGS. 2A to 2C, the arm link 130 and the shaft 120 are rotated by force transferred by the extension of the gas lift 110 in the length direction, such that the door body 200 is rotated to perform an opening operation or a closing operation. In the flap door for a vehicle according to the related art as described above, since the arm link 130 has a predetermined length and has the force applied to an end portion thereof (an acting point of the gas lift 110) through the gas lift 110, the length between the shaft 120 and the acting point of the gas lift 110 (a connection point between the gas lift 110 and the arm link 130) is not changed at the time of a rotation operation of the door body 200.
In the flap door for a vehicle according to the related art having the above-mentioned configuration, as a result of comparing a moment acting on the arm link 130 when starting an opening operation of the door body 200 in a state in which the door body 200 is closed as shown in FIG. 2A, a moment acting on the arm link 130 during the opening operation or a closing operation of the door body 200 as shown in FIG. 2B, and a moment acting on the arm link 130 when starting the closing operation of the door body 200 in a state in which the door body 200 is opened as shown in FIG. 2C, it has been confirmed that the moment when starting the opening operation of the door body 200 and the moment when starting the closing operation of the door body 200 are larger than the moment during the opening operation or the closing operation. Therefore, the flap door for a vehicle according to the related art has a problem that a user should apply large force to the door body 200 in order to open the door body 200 in the state shown in FIG. 2A or close the door body 200 in the state shown in FIG. 2C.